Here's where I think I'm headed on this swap. Griffith's Gnat variant; over-hackled / red tail; #22. I just like red on flies.
BlueDun, I've never added weight to a fly smaller than a #18. I'm sure it can be done, but it will also add bulk to the body of the fly. I'd probaby go with the smallest diameter lead (or non-lead substitute) that I could find. Others on this forum will know much better then I.